Trickling filter wastewater processes include the step of passing wastewater in a downward flow system in contact with biomass attached to a filter medium. A sufficient contact time between the wastewater and the filter medium is provided for the adsorption of soluble and colloidal material into the biomass. As a result of the oxidation or oxidative respiration step, new biomass is created. In addition, the biomass is reduced by endogenous respiration.
Trickling filter wastewater treatment processes are easy to operate and maintain, and are considered energy efficient relative to activated sludge processes because they do not need an expensive air supply. However, effluent quality is not consistently high, typically containing 20 to 40 mg/l of biochemical oxygen demand (BOD) and suspended solids (SS).
Although trickling filters were the most frequently used secondary wastewater treatment process until the 1940's, the application of trickling filters has gradually decreased in recent years due to an inability to meet a 30 day-30 mg/l BOD.sub.5 and SS standard. This is mainly due to a high effluent SS. In order to provide a high quality effluent, efficient removal of solids from the trickling filter effluent is necessary.
Many process modifications have been used to improve the performance of trickling filters. One such alternative is to replace the trickling filter with the activated sludge process or a rotating biological contactor (RBC). A second alternative is to use a tertiary treatment process such as filtration or chemical treatment to polish the effluent of existing trickling filter plants. A third alternative is to replace the existing rock media with plastic media to enhance the performance of the trickling filter. A fourth alternative is to modify the trickling filter process with a coupled activated sludge process. Each of these alternative measures can provide the means to meet the current secondary effluent discharge limitation of 30 mg/l of BOD.sub.5 and SS, but each is associated with additional capital and operating costs.
In the early 1980's, Norris et al. [1, 2] and Fedotoff [3] suggested a simple process modification for the trickling filter process to produce a high quality effluent without requiring expensive tertiary or coupled treatment. In this modification, the trickling filter effluent is mixed with return sludge from the final settling tank and further treated in an aeration tank or channel with a short hydraulic retention time. The aerated solids contact sludge is then settled in the final settling tank. These modifications are generally referred to as the Trickling Filter/Solids Contact (TF/SC) process.
The advantages of the TF/SC process modifications of trickling filters according to Norris et al. [1, 2], Fedotoff et al. [3], and Niku et al. [4], include: (1) lower capital cost than full scale activated sludge processes and the rotating biological contactors (RBC), (2) lower operating and maintenance costs, (3) simplicity of operation, (4) ease of biological sludge settling, (5) adaptability to existing trickling filters, and (6) equivalence of performance to the activated sludge process.
TF/SC plants have consistently produced an effluent quality which exceeds that of secondary treatment or comparable tertiary treatment plants. The production of high quality effluent is related to the enhanced flocculation and soluble organic removal property of the aerated solids contact sludge. However, the process kinetics and design parameters of the solids contact step are not fully understood.
As in other biological wastewater treatment processes, the efficiency of treatment attained by the trickling filter is greatly affected by the performance of the final settling tank. Most of the dissolved organic matter and colloidal solids in wastewaters applied to trickling filters are rendered settleable by adsorption and biological flocculation on the trickling filter biological film. The film itself is modified by decomposition and the net removal of solids in wastewater is varied and related to the biomass holding capacity in the filter. In spite of its importance, information on the sludge settling step in the trickling filter is extremely limited compared to that of the activated sludge process.
It is known that the flocculation of biological sludge is affected by various physical, electrochemical, and biochemical factors. The physical factors include the size of floc, degree of agitation in the system, surface area of floc, bound water, and solids concentration. The electrochemical factor includes the surface charge of flocs. The polymer content in the sludge represents the biochemical factor.
Although flocculation is closely related to the sludge settleability, there is no direct way to substantiate the degree of flocculation in the biological sludge. Sludge volume index (SVI), which represents the sludge settleability, is actually a measure of settled sludge volume as a result of the complex flocculation and compaction interaction during the settling process. Although the importance of polymeric material to sludge settleability has been recognized in the activated sludge process, study of the effect of polymeric material on sludge settleability in the trickling filter process has been limited.